Although the National Sleep Foundation recommends 8.5 to 9.25 hours of sleep per night for adolescents, more than half of US 16-year-olds regularly get less than seven hours. Sleep debt has consequences that extend far beyond hampering day-to-day functioning as it is deleterious to multiple areas of physical and mental health. In the context of the obesity epidemic, which has had a far-reaching public health impact, sleep insufficiency is emerging as a potentially important and novel risk factor for the development of excess weight gain and weight gain-related behaviors. While a variety of forces such as screen time, homework expectations, and other factors can be implicated in stymying American teenagers' sleep, research has shown that early high school start times, which are asynchronous with adolescent biology, are one of the most significant obstacles to youth being able to net sufficient sleep. When children begin puberty, circadian rhythms shift such that melatonin is secreted later in the evening making it difficult to fall asleep before 11pm. This neurobiological change means that adolescents who attend early-start (prior to 8:30am) high schools, even those who strive for an early bedtime, will struggle to get sufficient sleep. Natural experiments have clearly shown that later school starts allow adolescents a greater opportunity to accumulate adequate sleep, but unfortunately 86% of US schools start before 8:30am. We hypothesize that later start times, an established effective sleep intervention, may promote weight-related health. Given that adolescence is a critical age that sets the stage for long-term obesity risk behavior patterns, there is an urgent need to understand the obesity-related implications of increased sleep as a result of intervention and policy changes. Presently there is a unique opportunity to study a community- based natural experiment in school start time policy modification. A Minneapolis-St. Paul, MN metro area school district, will be shifting the start tim of its high school from 7:30am to 8:30am in Fall 2016. We propose to collect data on student weight and related risks (via survey, objective weight and height measurement, and sleep actigraphy) before and after this change in this district and high school students in a comparison district which will not be altering its 7:30am high school start time. This research is time-sensitive as baseline, pre-policy measures must be collected before the end of the 2015/16 school year. Our specific aims are: 1) Determine how a shift to a later high school start time relates to objectively measured weight change over time. 2) Identify the relationship between school start times and obesity-related behaviors over time. We have an additional exploratory aim to assess barriers and facilitators associated with implementing later high school start times via stakeholder interviews. The products of this research will have public health impact because they will clarify causal connections between sleep and obesity among adolescents as well as provide evidence for whether a school policy can minimize unhealthy weight gain. If effective, later school start times, a policy deployable in communities across the US, could help prevent excess weight gain in youth.